In the manufacture of cellulosic pulp from wood chips, wood chips are subjected to high-temperature conditions to separate lignin from cellulose fibers. The lignin is then solubilized in a cooking medium. There are three main chemical treatment processes that are currently used in the manufacture of cellulosic pulps such as acid sulfite, soda and kraft treatment processes. Soda and kraft processes are accomplished at high pH while the acid sulfite treatment is done under acidic conditions.
In all three of these processes the step subsequent to cooking the lignin is the step of washing the pulp to separate dissolved lignin from fibers. Because lignin is a complex organic molecule, products of the cooking step can create fragments that are surface active, which helps stabilize foaming. Additionally, other organics originating from resins in the wood, such as fatty acids and sterols, are also present and are potent foaming agents and can thus create issues in the pulp washing process due to excessive foaming.
The unit operation of washing cellulosic fiber uses a combination of dilution and displacement washing. In order to use the least amount of water or other rinse fluid, good mixing is required, which often results in foam formation. This foam has negative consequences to the washing operation. The foam fills the process equipment and reduces capacity in the washer vats and filtrate tanks for the black liquor and the air bubbles in the fiber slurry get trapped in the fiber mat which is formed during washing and prevents efficient washing of the pulp. For these reasons brownstock washing operations typically use wash aids as a process aid that is fed at different points of the washing operation. For one skilled in the art it is recognized that brownstock washing operation is any process or unit operations between the digestion step to final pulp storage where the dissolved, colloidal or suspended solids are separated from the cellulosic fiber. It is envisioned that the current method can be used not only with all types of Brownstock washers including displacement washers and rotary vacuum washers, but also wherein the cooked chips, which are not yet disintegrated into individual fibers, are washed within the digesters, examples of which are pressure diffusers. The present composition can also be added after other delignification steps for the cellulose, such as in oxygen/ozone/peroxide delignification and other the bleaching and/or extraction stages.
Conventional defoamer products are water-based emulsions and contain emulsified droplets of active defoaming ingredient within a continuous water matrix. The method by which these drops affect defoaming is well known and is described in Langmuir, vol 15, pg. 8514, 1999. The composition of the active ingredient, known and practiced in the art, can be quite varied. The substantially water-free fluids of this invention can comprise, but are not limited to, any of these compositions. One class of active defoaming composition is paraffinic oils or mineral oils, which are typically mixtures with a wide range of carbon chain lengths. Although lower carbon chain length molecules can be effective, they are typically avoided due to VOC and environmental issues. Therefore, most mineral oils are restricted to have molecules with carbon chain length greater than 12. In addition to the mineral oil, hydrophobic particles need to be incorporated into the mineral oil. Typically, hydrophobically modified silica particles or particles of wax such as ethylene bis-stearamide are used, although other types of hydrophobic particles may also be employed. The hydrophobic particles are typically used at a level of 2%-30% in oil phase. Other oils that can be used as the substantially water-free fluid include vegetable oils which are mixtures of tri-glycerides, long-chain alcohols (C>10), polypropylene glycol, and polyethylene/polypropylene copolymers.
There are several types of agents used as brownstock wash aids and can be classified by their chemical composition. Mineral oil based wash aids are typically used as defoamers, and typically contain hydrophobic particles like silica or ethylene bis-stearamide. These were the most common type of wash aid or defoamer a few years ago. However, with the introduction of organo-silicone based wash aids, their use has decreased. The simplest form of organo-silicone wash aids or defoamers are mixtures of polydimethyl siloxane (PDMS) fluid containing hydrophobic particles, such as hydrophobic silica. Substantially, more efficient wash aids typically used as defoamers or antifoam agents are organo-silicones that are modified. U.S. Pat. No. 5,380,464, discloses branched siloxanes in combination with polyether/siloxane copolymers. Mixtures of siloxanes with branched or crosslinked polyether/polysiloxane copolymers, optionally containing a filler such as silica are taught in U.S. Pat. No. 6,605,183 and U.S. Pat. No. 6,512,015. EP Patent Application No. 163,541 disclose silicone-based defoamers in which PDMS is replaced with silicone resin, which is a partially crosslinked organo-silicone compound. U.S. Pat. No. 7,645,360 and U.S. Pat. No. 7,550,514 disclose incorporating aliphatic groups within the silicone compound and U.S. Pat. No. 7,619,043 discloses incorporating phenyl groups within a compound. U.S. Pat. No. 6,512,015, U.S. Pat. No. 7,645,360, disclose organo-silicone compounds containing polyethers—ethylene oxide and propylene oxide are mixed with the silicone resin made with PDMS and silica.
While the organo-silicones are able to perform quite well when added directly to a pulp slurry during the washing step, there are several advantages to being able to emulsify hydrophobic silicone materials into water and then introduce them into the pulp slurry. Their effectiveness is enhanced, and the potential to deposit is reduced. Consequently, the organo-silicone resins are sold as prepared emulsions. While these emulsions have an enhanced effectiveness there are issues that arise in the manufacture and distribution of these products. Organo-silicones are very difficult to emulsify and stabilize over a long period of time. Because of their lower density, the emulsion drops tend to rise to the top (cream or phase separate) over time. The term emulsion is used to refer to a two phase system with liquid droplets in a continuous liquid medium. This can be mitigated by adding viscosifiers, but the dispersibility of these emulsions into the pulp stream becomes more difficult. Viscosifiers or thickeners are often gums which will increase the propensity to be contaminated by microbial organisms. Also, because emulsions are hard to stabilize, they are more prone to coalescence and consequently it is harder to make concentrated emulsions. Typically emulsions have to be prepared at less than 50% actives and more typically they are around 30% actives. Because of the high volume of water in these emulsions the cost to make (larger vessels) and transport them to the site they are being used at, based on the level of actives, is cost prohibitive.
Therefore, it would be desirable to be able to make and ship a product that is 100% active and to emulsify the wash aids at the customer site with a simple and inexpensive method, Several previous attempts have been made to emulsify organo-silicones on-site. One method is to make the organo-silicone material self emulsifiable. Chinese Patent Application No. 10/2174778 discloses a formulation to make a PDMS/hydrophobic silica mixture self emulsifiable by incorporating silicone polyether (SPE) and emulsifiers such as ethoxylated alcohols. Japanese Patent No. 2000/246010 and Japanese Patent Application No 08/309104 disclose compositions that are alleged to be self-emulsifiable. While self-emulsifiable compositions can aid in the emulsification of wash aids or defoamers, they present several issues. One issue is that the sizes of the drops still depend on the shear rate present in the process stream, which is not always controllable. Another issue is that the ingredients used to make the silicone self-emulsifiable can affect the performance of the silicone as a defoamer. There have been some attempts to enhance the performance of defoamers by enhancing the mixing at the feed point in the customer process. U.S. Patent Application No. 2010/0300632A discloses a mixing valve to enhance the mixing at the feed points for many different additives, but would not be very effective at emulsifying organo-silicones. U.S. Pat. No. 6,162,325 discloses mixing two streams of process aids wherein at least one defoamer and one emulsifier is mixed and fed to a washer. As the defoamer is delivered and added to the wash system in “neat” form, the emulsifier is mixed with the defoamer in an amount of from about 1% to about 20% by total weight of the defoamer composition prior to the defoamer/emulsifier composition being injected into the washer. Since the defoamer added to the system is added “neat” or not pre-emulsified, the emulsifier enhances the ability of the defoamer to disperse into drops defoamer once it is introduced to the washer. While the improvement in dispersibility is desirable, it is not always sufficient because the drop formation also depends on the shear applied and each washer and the defoamer feed points offer different levels of shear, some of which may not be sufficient for desired drop size.
All references cited within this application are incorporated herein in their entirety.
There exists a need to be able to emulsify a substantially water-free silicone wash aid at the site where it can be used soon after its emulsification in the application of interest. Furthermore, it has been found that two or more brownstock washing aids having different performance characteristics, when simultaneously mixed and emulsified can achieve the desired defoaming and other characteristics can be realized in real time.